WhatsApp Messenger allows you to send free text messages over the internet, saving you the cost of sending SMS.
You can use WhatsApp Messenger to send messages over WiFi or 3G to anyone else with the app installed on their phone. The software works across mobile operating systems (iPhone, Android, BlackBerry, etc.) so it will function with contacts who have other versions of WhatsApp Messenger installed.
The setup procedure for WhatsApp Messenger is a bit fiddly and it can take a few minutes to get it up and running. Once set up, though, it's pretty easy to use. The chat window displays the history of the conversation in linear form. Just type and hit Send to post the message.
WhatsApp Messenger gives you a number of other handy options aside from just sending text. You can attach a picture (taken from your camera or stored on your device), voice note, video, contact, and even your location. There's also an option to email your chat history.
The catch with WhatsApp Messenger is that it only works if both people in the conversation have the app installed.
Overall though, WhatsApp Messenger makes for a handy way to chat for free on your BlackBerry.